Question: The arithmetic sequence $(a_i)$ is defined by the formula: $a_i = 3 - 6(i - 1)$ What is $a_{13}$, the thirteenth term in the sequence?
Solution: From the given formula, we can see that the first term of the sequence is $3$ and the common difference is $-6$ To find $a_{13}$ , we can simply substitute $i = 13$ into the given formula. Therefore, the thirteenth term is equal to $a_{13} = 3 - 6 (13 - 1) = -69$.